<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        
       .div 

        body{
            display: flex;flox-flow:column;
        }
        .head,.foot{
            height: 200px;border:1px dashed blue;
        }
        .body{
            flex:1;display: flex;
        }
        .side{
            width:200px; height: 200px;border: 1px dashed blue;
        }
        .main{
            flex: 1;border: 1px bashed blue
        }
    </style>

</head>
<body>
    
    <div class="head">上</div>
    <div class="body">
        <div class="side">左</div>
        <div class="main">右</div>
    </div>
    <div class="foot">下</div>
</body>
</html>